Modern day electronic systems are compact and often include one or more high-power, high-density devices, such as microprocessors or other chip sets or chip packages. As the functionality of these microprocessors increases, the circuitry in the processor is becoming denser and operating at higher speeds which results in more heat being produced. The high-power, high-density chips frequently cannot be adequately cooled by the conventional forced air cooling system used to cool the overall electronic system. Instead, these chips require their own, i.e., dedicated, cooling systems. These dedicated cooling systems may include a heat sink, a fan, a heat spreader, or a heat pipe.
The purpose of a heat pipe is to move the heat from the point of generation to a remote location for dissipation. A heat pipe is a sealed, usually evacuated chamber which contains a liquid coolant. The liquid coolant or working fluid changes phase as it absorbs and dissipates heat. The coolant changes from liquid to vapor as heat is transferred to it from the chip, and changes from vapor back to liquid as it dissipates the heat to the surrounding environment.
The end of the pipe which acts as the evaporator is placed in thermal contact with the chip or other heat source. The heat conducts through the walls of the heat pipe to the working fluid. This causes the working fluid to vaporize. The vapor travels to the condenser end of the heat pipe where the vapor is cooled by the surrounding environment causing it to condense back into its liquid phase, thus liberating the heat. A wicking structure or other capillary device draws the condensed working fluid back through the pipe to the evaporator, where heat is again transferred to it from the chip.
Conventional heat pipes are relatively rigid and may include tubular expandable bellows to permit bending of the heat pipe when it is installed in an electronic device. Because these heat pipes are made out of rigid materials such as copper tubes or pieces of sheet metal, they can have relatively thick effective cross-sections. These heat pipes can be bent into their final fixed shape. Generally, large bending radii are required to enable the heat pipe to function properly. These conventional heat pipes thus can not be readily and repeatably deformed, either elastically or plastically, to match the space constraints of the electronic system. In addition, repeated plastic deformation could lead to embrittlement or fatigue of the heat pipe which would result in reliability failures that would shorten the useful lifetime of the heat pipe.
In laptop computers, the central processing unit (CPU), memory chips or other computer chips are typically attached to a printed circuit board located in the base portion of a laptop computer. The heat generated by the chips is either dissipated through the base portion of the laptop computer which can get uncomfortably warm if it is sitting on the user's lap or a fan may be used to move the heat away from the heat generating chips.
Rather than dissipating the heat through the base portion of the laptop computer, it would be advantageous to dissipate the heat to the backside of the screen in the cover of the laptop computer. This requires the heat pipe to pass through the hinge that joins the cover portion to the base portion. In this application, the heat pipe must be flexible to allow for repeated opening and closing of the cover. A current method of addressing this application is to utilize two separate rigid heat pipes connected to a flexible heat sink, typically two hinged blocks of aluminum. The first heat pipe moves the heat from the chip or CPU to the flexible heat sink which transfers the heat through the hinge to a second heat pipe or a heat spreader. The weight of this heat sink assembly, as well as having at least three heat transfer interfaces, makes this approach less desirable for most portable or hand held devices.
Additionally, as consumers demand smaller, more powerful systems, providing space for these rigid heat pipes becomes more of a problem. A need exists for a heat pipe that can fit into relatively narrow spaces and can be flexed repeatably.